Wide bandwidth oscillator/counter A/D converter

We present theory and performance data on oscillator/counter AID converters fabricated in TRW's Nb process. Circuits with junction critical current de nsity of 2 kA/cm(2) are discussed, This simple, low power A/D converter arc hitecture is uniquely enabled by the wide bandwidth voltage-controlled osci llator (VCO) and compatible, ultra-fast superconductor single-flux-quantum (SFQ) divide-by-two flip-flops, The measured signal-to-noise ratio (SNR), f requency response, and signal distortion are discussed within the framework of a basic theory of performance as well as time-domain simulations. Noise contributions from quantization error, aperture jitter, and thermal noise are included. The measured signal-to-noise ratio (SNR) is shown to be withi n a few dB of theoretical performance over more than two decades of frequen cy. The frequency response is shown to fit well to the expected function th rough I GHz of signal frequency. Harmonic distortion is shown to be consist ent with the non-linearity in the front-end's DC I-V characteristic which i s subject to design improvement, This architecture extends to higher perfor mance using a multi-junction VCO, Measured two-junction VCO SNR shows essen tially the theoretical improvement over that of a single junction.